Coxiella burnetii is an obligate intracellular pathogen and the cause of Q fever. Domestic animals are the primary reservoir, thus, people involved in animal handling are at risk of natural infection. Because C. burnetii is highly infectious and remains infectious in the environment, it has been weaponized in the past, and is a current bioterrorism risk. The LPS structure of C. burnetii is a known virulence factor and the interaction between C. burnetii and the LPS receptor, toll like receptor 4 (TLR4), has been studied by multiple groups. Mouse and human TLR4s bind LPS differently, and the functional responses are distinct in response to some agonists. For example, hypoacylated LPS from Yersinia pestis induces robust inflammatory responses in mouse cells, but dampens such responses in human cells. The LPS of C. burnetii is similarly hypoacylated and suppresses inflammation in human monocytes/macrophages, however this has not been examined in vivo. We utilized a novel human TLR4/MD2 transgenic mouse to demonstrate that human TLR4 (hTLR4) enhances infection with C. burnetii relative to mouse TLR4 (mTLR4). Type I interferon (IFN) signaling is an important pathway for innate protection from infection with viruses and many bacteria, such as Legionella pneumophila, which is closely related to C. burnetii. We have recently found that rather than promoting protection from C. burnetii, type I IFN signaling promotes disease induced by C. burnetii. The intent of this proposal is to investigate the potential link between enhanced susceptibility of hTLR4-expressing mice and type I IFN signaling. The following hypothesis will be tested: Type I IFN promotes C. burnetii pathogenesis in mice expressing human TLR4. The following Specific Aims will be pursued: Specific Aim 1) Determine the impact of type I IFN signaling in the disease course for C. burnetii in hTLR4/MD2 mice. Specific Aim 2) Determine the role of type I IFN signaling in the generation of permissive and restrictive macrophage subsets in hTLR4/MD2 mice. At the conclusion of this project we will have characterized an innate immune pathway that has potential for development of novel therapeutics for use in Q fever. Because of the novel hTLR4/MD2 mouse model, the results will be more readily translated to human disease.